Details
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Task
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Resolution: Won't Fix
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Minor - P4
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None
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None
Description
The bitmap option with Linux Software Raid (i.e. md) is dangerous when used with MongoDB.
To figure out if you're using bitmap, see bitmap in /proc/mdstat that the raid array was set up with this
feature.
While "bitmap" can improve recovery time for large RAID arrays, it can have disastrous effects on write performance because it needs to update the bitmap before it can do any write. This is particularly problematic when using journaling.
This can potentially double the IOPS requirements (tripling if you need to read-modify-write the bitmap.) It also means that otherwise sequential I/O becomes non-sequential. To disable bitmap use the mdadm /dev/mdX --grow --bitmap none command.
Immediately after disabiling bitmap, the io utilization on the server will reduce and throughput will stabilize.